Traditional rolling element bearings made from AISI 52100 bearing steel or M50 high-speed steel are limited in performance for use at very high speeds, high temperatures, and corrosive environments. They need adequate lubrication for satisfactory operation and optimum life. Over the past decade or so, silicon nitride (Si3N4) balls have become an important component in advanced bearings. They are most successfully used in hybrid bearings (SNHB). Si3N4 is typically the preferred material because it offers many desirable properties, such as high hardness, high thermal and chemical stability, low density, high Young's modulus, high stiffness, good fatigue life, low friction, and high wear resistance.
The lower density of Si3N4 balls reduces the gyroscopic slip and centrifugal loading on the outer steel race. This reduces friction, heat, and wear of the bearing elements. Si3N4 balls do not react with the steel race and hence, micro-welding can be avoided resulting in longer bearing life. SNHB are well suited for applications where marginal lubrication is required and are less sensitive to the lubricant type, lubricant contamination, and lubricant starvation. Higher rigidity, excellent surface finish and sphericity of Si3N4 balls reduces noise and vibrations of these bearings, thus enabling higher speeds.
The benefits listed above make Si3N4 hybrid bearings (SNHB) suitable for high speed and high temperature applications, such as turbines, machine tool spindles, dental drills, liquid oxygen pumps, and turbo molecular pumps. All-ceramic bearings (Si3N4 balls in Si3N4 races), on the other hand, can operate at higher temperatures, far outside the range of SNHB (−40 to +200° C.). They perform well in aggressive environments such as semiconductor processing, infrared missile seekers, and tidal flow meters. However, their use is limited mainly due to the difference in the thermal expansion coefficient between the metal drive shaft and the inner ceramic race, and hence, SNHB are preferred for most other applications.
Compared to traditional steel bearings, SNHB, in general, can more easily meet the requirements of higher efficiency, higher reliability, higher accuracy, higher speed, greater stiffness, longer life, lower friction, corrosion resistance, marginal lubrication, and lower maintenance costs.
The failure mode of Si3N4 balls is by fatigue flake-off which is similar to metallic rolling elements due to its higher fracture toughness. The performance and reliability of ceramic rolling element bearings depend on the quality of the resulting surface. Ceramics have high hardness and inherent brittleness. They are sensitive to defects resulting from grinding and polishing processes. Fatigue failure begins at regions of surface irregularities, such as scratches, pits, and microcracks. Hence, it is important to produce superior quality and finish with minimum defects in order to obtain reliability in performance.
In industry, ceramic balls have heretofore been finished by conventional grinding followed by V-groove lapping. This is essentially the same technique used for finishing steel balls. The balls run in 3-point contact in a V-groove. The balls revolve around the pad and at the same time rotate continuously. They glide and roll relatively against the contacting surfaces of the pad. The process uses high loads (about 10 N per ball), low polishing speeds (about 50 rpm), and a diamond abrasive. Due to lower speeds, considerable time (6-16 weeks) is required for finishing a batch of ceramic balls from the as-received condition to the finished condition. Thus, the long processing time and the use of expensive diamond abrasive result in high processing costs. Furthermore, the use of a diamond abrasive under heavy loads can result in scratches, pits, and microcracks on the surface and subsurface of the polished balls. These surface defects can act as nucleation sites for cracks resulting in catastrophic failure by large brittle fracture.
In order to prevent such failures, it is necessary to minimize the surface damage as much as possible. For this purpose, gentle polishing conditions are required, namely, a low level of controlled force and abrasive which are not significantly harder than the work material. High material removal rates and shorter polishing times can be obtained using high polishing speeds. This is accomplished by a process known as magnetic float polishing (MFP).
MFP processes typically use low loads (about 1 N per ball), high speeds (about 2000 rpm with a 2.5 inch diameter upper part of the chamber in the small batch apparatuses and about 500 rpm with a 12.2 inch diameter upper part of the chamber in large batch apparatuses), and abrasives such as B4C, SiC, and CeO2. For ¾ inch ceramic balls, a typical small batch apparatus will process a batch of about six balls whereas a large batch apparatus will typically hold a batch of about 46 balls. Similarly, 10 balls of ½ in. diameter and 15 balls of ⅜ in. diameter can typically be finished by a small batch apparatus as compared to about 69 balls of ½ in. diameter and about 104 balls of ⅜ in. diameter in the large batch apparatus. An actual polishing time of 20-30 hours is typically required to finish a batch from the as-received condition.
The processing time is not affected by the number of balls used in a given apparatus. For example, a modified float chamber having multiple ball tracks or rows can take 100-200 balls of ¾ in. diameter instead of 46 used in the single track apparatus, and yet, it will take about the same amount of time to polish them.
Magnetic float polishing MFP is based on the magneto-hydrodynamic behavior of a magnetic fluid that can levitate all non-magnetic materials suspended in it. A bank of permanent magnets (Nd—Fe—B) is arranged with alternate N and S poles below an aluminum chamber filled with the required amount of magnetic fluid and an appropriate abrasive (5 to 10% by volume). The magnetic fluid (also called ferrofluid) is a colloidal dispersion of extremely fine (100 to 150 Å) sub-domain ferromagnetic particles, usually magnetic (Fe3O4) in a carrier fluid, such as water or hydrocarbons (e.g., kerosene). Water in the magnetic fluid not only acts as a coolant but also participates in the chemical reaction with the work material during the polishing process. The ferrofluids are made stable against particle agglomeration by the addition of surfactants.
When a magnetic field is applied, the Fe3O4 particles are attracted downwardly towards the area of higher magnetic field and a resultant upward buoyant force is exerted on all of the non-magnetic materials, to push them upwardly toward the area of lower magnetic field. The abrasive grains, ceramic balls, and the acrylic float inside the chamber, all being non-magnetic materials, are levitated by the magnetic buoyant force.
The magnetic float polishing chamber is preferably installed in and operated using a machining tool (e.g., a Bridgeport vertical machining center). An upper piece of the polishing chamber is lowered into a lower chamber piece to make contact with the balls and to press them down to reach the desired level of force or height. A piezoelectric dynamometer, placed between the chamber and machine tool table, is used to measure the exact loading. The balls are polished by the abrasive grains under the action of the magnetic buoyancy levitational force when the upper piece of the polishing chamber is rotated by the machine tool spindle. A damage-free surface on ceramic balls is expected by the magnetic float polishing technique because the magnetic buoyant force (typically about 1 N/ball) is applied via a flexible acrylic float positioned in the polishing chamber beneath the workpieces. The function of the acrylic float is to produce a uniform, larger polishing pressure. A urethane rubber sheet is glued to the inner guide ring of the polishing chamber to protect it from wear. The material of the upper piece of the chamber is non-magnetic, austenitic stainless steel.
Once the balls are reasonably uniform in size, uniform removal of material from the ball surface is essential for obtaining good sphericity. Heretofore, this has been particularly problematic in large batch apparatuses, especially with large balls having large surface area, which increases as the square of the diameter of the balls. The apparatus and set up procedures currently used in the art are not capable of consistently and efficiently achieving the precise required geometric alignment and coaxially of the upper and lower polishing chamber pieces with each other and with the powered machine tool spindle.
The coaxiality of the upper piece of the polishing chamber with the machine tool spindle is particularly important but has been very difficult to accomplish. Heretofore, the upper chamber piece has been formed using precision machining techniques and then secured to the drive spindle of the machine used for the ball polishing operation.
Establishing and maintaining coaxiality between the upper and lower pieces of the polishing chamber has proved to be even more difficult to achieve and is one of the most significant factors affecting the results. If the upper and lower pieces of the polishing chamber are not aligned properly, unequal loading will result, which will cause higher material removal rates at areas of higher loading. This means that, as a ball circulates around the chamber, some areas of the ball's surface will be machined more than others, thereby severely degrading the ball's sphericity. Also, this unequal loading is a source of vibration, which again has the same effect.